The following works because you can write table formulas with elisp code.  Notice the `'` (quote) before the s-exp.

```
| # | B | C |
|---+---+---|
| 1 | 2 | 3 |
| 1 | 2 | 3 |
#+TBLFM: @1$1='(format "#")
```

Evaluable `(info "(org) Formula syntax for Lisp")` for more information.